I can't define a significant difference in meaning betwen "detailed" and "detached". Either way, a member of a military unit has an assignment that places him away from his command. Could be that "detailed" is for temporary purpose, while "detached" is a long-term, indefinite assignment.

Eventually some Confederate soldiers assigned to duty as pioneers recived transfers to CS engineer regiments. They built bridges, fortifications &c. Unlike 20th century military engineers, these men were not equipped or used in combat situations.

Similar unit titles included Sappers and Miners. There should have been at least one company per division, but this wasn't done in the Army of Tennessee until 1864.